BIRMINGHAM, Ala. — Envelopes containing white powder set off anthrax scares in five Alabama cities, shutting down two federal courthouses Monday and trapping a congressman in his office as authorities tested the substance.

No one was injured, and investigators said at least five of the letters — all sent to the offices of senators or congressmen — were believed to be from the same source.

None of the letters tested positive for anthrax or other hazardous substances, but officials didn’t immediately say what they contained.

Authorities said letters containing white powder were sent to the Mobile and Foley offices of Republican Rep. Jo Bonner, who was forced to remain with staffers in the Mobile office while officials tested the substance.

“Each letter contained a small bag with a white powdery substance, and neither of these bags were opened,” said Mike Lewis, a spokesman for Bonner.

The FBI and U.S. Postal Inspection Service said letters also were sent to the offices of U.S. Rep. Mike Rogers and U.S. Sens. Jeff Sessions in east Montgomery and Richard Shelby in the federal courthouse in downtown Montgomery. FBI spokeswoman Angela Tobon said all the letters sent to the lawmakers’ offices appeared to be from a common source.

Chuck Spurlock, Sessions’ state director, said employees notified the FBI of the suspicious letter and closed the office about noon. He said he received notification late Monday afternoon that the letter did not contain anthrax.

Shelby’s spokesman, Jonathan Graffeo confirmed receipt of the letter, but said he had been asked by the FBI not to comment.

Rogers’ press secretary Shea Snider said a letter was also sent to Rogers’ Anniston office. Snider said the Anniston office was reopened after authorities determined the powder was not dangerous. She said the powder from the Montgomery office was still being tested late Monday. She said no one was injured in either of the offices.

Two more letters were sent to the federal courthouse in Anniston, and another letter was found at a federal courthouse in Birmingham, where an employee doing extra work found the first envelope Sunday at the Robert Vance Federal Building.

“It wasn’t anything that was dangerous,” said Dr. Donald Williamson, head of the Alabama Department of Public Health.

With the Vance building already closed through Wednesday as a precaution, authorities were called to the federal courthouse in Anniston on Monday morning after mysterious powder was found in an office there, prompting another shutdown.

“We did sample it and test it, and it came back negative for a hazardous material,” said Anniston Fire Chief Bill Fincher.

Foley Fire Chief Joey Darby said the letter found at Bonner’s office there tested positive for “a very common household product,” but he wouldn’t elaborate. The letter apparently was delivered last Thursday and sat unopened during the long New Year’s holiday weekend.

“From what we are hearing that’s consistent with some of the other cases,” he said.

Williamson said tests would be conducted to determine the exact nature of the powder. Five people who were exposed to the powder in Birmingham were given antibiotics as a precaution.

Postal inspector Tony Robinson said the agency was offering a $100,000 reward for information leading to an arrest.

Anniston is about 65 miles east of Birmingham in northern Alabama; Montgomery is in the central part of the state; and Mobile and Foley are on the coast.